1. Field of the Invention
The present invention relates to a shift control device for a hybrid vehicle having a plurality of power sources such as an engine or a motor mounted thereon, and more particularly, to a shift control device which is enabled to perform a continuously variable transmission by differential mechanisms such as planetary gear mechanisms.
2. Description of the Related Art
A hybrid vehicle using a differential mechanism is disclosed in JP-A-2000-142146.
JP-A-2000-142146 discloses a planetary gear including three elements (sun gear, carrier and ring gear), and if for any two elements the rotating speeds are determined, the rotating speed of the remaining element is determined. This is called three elements and two degrees of freedom. These three elements are connected to an engine, an output to wheels and to two motor/generators.
In this hybrid vehicle, a part of engine power is supplied to one of the motor/generators to generate electric power, and the electric power is supplied to the other of the motor/generators to work as a motor. The motor carries out working as a continuously variable transmission and increases or decreases the output driving power by using the differential mechanism.
However, in the construction of JP-A-2000-142146, one of the motor/generators is connected to a drive axle directly, and there is only a final gear ratio between the motor/generator and wheels. So it is often necessary to use a battery power. Also an area wherein it is possible to drive the motor/generator by only a power that the engine generated is narrow.
Especially while accelerating, a driving torque and an electric power consumption of the motor/generator increases. And if the operating region wherein the motor/generator consumes a battery is increased, it is necessary to provide a large capacity of the battery.
In the present invention, there is provided a shift control device for a hybrid vehicle having a differential mechanism including at least four rotating elements whose rotating states are determined by two element rotating states. The differential mechanism can amplify an output torque. In this hybrid vehicle, an engine connected to one of the rotating elements may be controlled for optimum fuel efficiency, because of fuel efficiency.
But if the engine is always controlled for optimum fuel efficiency, when high acceleration is required, driving power will be lacking.
The present invention provides a solution to the above mentioned problems of a large capacity of the battery and lacking a driving power.
According to the present invention, there is provided a shift control device for a hybrid vehicle. The shift control device has a differential mechanism including at least four rotating elements whose rotating states are determined by two elements, and each of the elements is connected to a driving source, two motor/generators and an output.
And the shift control device has a controller programmed to calculate a target driving power based on a driving condition; make a first judgment as to whether the target driving power is less than or equal to a first driving power wherein the driving source is driven under optimum fuel efficiency calculated based on a current output rotating speed; when the first judgment shows that the target driving power is larger than the first driving power, calculate a first driving source speed and a first driving source torque wherein the target driving power is realized such that the driving source is driven for maximum load; and control the driving source under the first driving source speed and the first driving source torque.